Spectacle lenses are usually manufactured according to a prescription of an ophthalmologist or an optician. The ophthalmologist or optician, respectively, determines the ametropia of an eye of a customer and determines a prescription of a spectacle lens for the eye such that, when a spectacle including the lens is worn by the customer, the ametropia of the eye is compensated as far as possible. The prescription typically comprises values for a spherical effect and, if necessary, values for the astigmatic effect of the spectacle lens, wherein the astigmatic effect is defined by a cylindrical power and a direction of the axis of the astigmatism. The prescription may further comprise additional values, such as a prismatic power, an addition, which defines a difference between spherical powers at a near reference point and a distance reference point of a multifocal lens, or an interocular distance.
The prescription includes values of the astigmatic effect if the eye of the customer requires an astigmatic correction. The spectacle lens is then manufactured such that it provides the astigmatic effect at at least one reference point by providing a corresponding cylindrical power and direction of axis. A spectacle lens should compensate the astigmatism of the eye not only at one reference point but, if possible, across the whole surface of the spectacle lens.
The most simple way of correcting an astigmatism is to provide a surface of a spectacle lens with a toric shape such that the astigmatism of the eye is sufficiently corrected at least at a reference point or a region around this reference point.
It has been found that such correction of an astigmatism does not provide completely satisfying results for customers and, in particular, those customers requiring an astigmatic correction of more than 1 or 2 diopters.